(a) Field of the Invention
The present invention relates to a manufacturing method of a polysilicon thin film transistor.
(b) Description of the Related Art
Generally, a liquid crystal display (“LCD”) has two panels with electrodes, and a liquid crystal layer interposed between the two panels. The two panels are sealed to each other by way of a sealant printed around edges of the panels while being spaced apart from each other by way of spacers.
The LCD generates electric field in the liquid crystal layer interposed between the panels and having dielectric anisotropy by using the electrodes and adjusts the strength of the electric field to control the light transmittance, thereby displaying images. Thin film transistors (“TFTs”) are used for controlling signals transmitted to the electrodes.
A most usual TFT for utilizes amorphous silicon as a semiconductor layer.
The amorphous silicon TFT bears a mobility of about 0.5–1 cm2/Vsec. Such a TFT may be used as a switching element of the LCD. However, since the TFT has lower mobility, it is inadequate for directly forming a driving circuit on the liquid crystal panel.
In order to overcome such a problem, it has been proposed that the polysilicon bearing a current mobility of about 20–150 cm2/Vsec should be used as the semiconductor layer of a TFT for an LCD. As the polysilicon TFT involves relatively high current mobility, a Chip In Glass technique which incorporates driving circuits into a liquid crystal panel can be realized.
In order to form the polysilicon thin film, it has been proposed to employ a technique of directly depositing polysilicon on a substrate at high temperature, a technique of depositing amorphous silicon on a substrate and crystallizing the polysilicon layer at high temperature of about 600° C., and a technique of depositing amorphous silicon on a substrate and heat-treating the amorphous silicon layer using laser. However, such techniques require high temperature processing and hence, it becomes difficult to employ the techniques to a glass substrate for a liquid crystal panel. Furthermore, the uniformity in the electrical characteristics between the TFTs is deteriorated due to the non-uniform grain boundaries.
In order to solve such a problem, a sequential lateral solidification process which can control the distribution of the grain boundaries in an artificial manner has been developed. This is a technique based on the fact that the grains of the polysilicon grow perpendicular to the interface between the laser-illuminated liquid phase region and the non-illuminated solid phase region. The laser beam passes through a mask with slits to completely melt local portions of the amorphous silicon to form liquid phase regions in slit shapes at the amorphous silicon layer. Thereafter, the liquid phase amorphous silicon is cooled to be crystallized. The growth of crystal begins from the boundary of the solid phase region where the laser is not illuminated, and proceeds perpendicular thereto. The grains stop growing at the center of the liquid phase region while meeting there. Such a process is repeated while moving the mask slits in the growing direction of the grains so that the sequential lateral solidification can be made throughout the entire target area.
However, a protrusion portion is formed on the grain boundary surface of the polysilicon layer crystallized by the sequential lateral solidification process. Accordingly a photoresist film is not completely applied on an upper surface of the polysilicon layer. To solve the problem, an organic cleaning process, or a cleaning process using HF is practiced, but it is not effective because the protrusion portion is not completely removed.